Many forced air heating furnaces now in use are of the type in which the hot flue gases from the furnace heat exchanger are vented through a draft diverter and flue pipe to a chimney. The temperature of the flue gases from the furnace are usually relatively high, of the order of 450 degrees to 500 degrees F., and a large amount of heat in the flue gases is lost. Further, the draft diverter and flue system of the furnace draws air from the furnace room, even when the furnace burner is shut off, so that there is a continuing loss of warm air from the furnace room.
Various different apparatus have heretofore been proposed for recovering heat from the flue gases and reducing heat loss due to venting of furnace room air to the chimney when the furnace burner is shut off. However, there remains a need for a flue gas heat recovery apparatus which can be installed in the return air duct and flue pipe of an existing forced air heating system, to preheat the return air by the furnace flue gases and control venting of the furnace room air when the furnace burner is shut off, in a safe, reliable and efficient manner.